Shure M68 mixer
« on: July 13, 2019, 08:10:44 AM »
Just wondering if anyones repurposed the transformers from these very common units , 
They look very good quality ,mu metal sheilded , four are used as inputs and the fifth is in reverse as balanced output , 200 ohms input to hi z .


rackmonkey

Re: Shure M68 mixer
« Reply #1 on: July 13, 2019, 03:25:26 PM »
It’s been a while, but I’ve used those as well as the inputs from the M-67, M-267 and M-367 in prototypes/experiments.  IIRC they are very high ratio inputs for transistor gear, something like 1:20 or 1:26 or thereabouts. People who tear up the old Akai/Roberts reel to reels seem to use them a lot, since the guy that originally started talking them up on the old Tape Op forum recommended them.

I have a post here somewhere with a Google Drive link to a bunch of response curves of mostly oddball pulls from various pieces of 60s/70s gear, and all of the above (including the M68 inputs) are in there.

They aren’t bad response-wise if I remember right, but can’t take much level as you might expect.

Edit: link to Google Drive folder:

https://drive.google.com/drive/folders/0B__Vr0ik2GcgLVlidXRjd0c2b1U?usp=sharing
« Last Edit: July 13, 2019, 03:29:36 PM by rackmonkey »
Whether you think you can or you think you can't, you're probably right.

rackmonkey

Re: Shure M68 mixer
« Reply #2 on: July 13, 2019, 03:37:55 PM »
Looking back at the M68 curve, I remembered that those were linear plots. Later, I started saving the linear and the log plots. Log plots would look pretty flat out to 20khz on that transformer.
Whether you think you can or you think you can't, you're probably right.

TwentyTrees

Re: Shure M68 mixer
« Reply #3 on: July 13, 2019, 03:47:31 PM »
Yeah, I have a shoebox full of those  bad boys!  Anywhere you need a significant step up, they work at a pinch for prototyping. I’ve even used them for line level on occasion, keeping levels (very) conservative.

If memory serves, the M68 inputs are about 1:10, M67s are 1:14 or so.

TwentyTrees

Re: Shure M68 mixer
« Reply #4 on: July 13, 2019, 04:01:48 PM »
I’ve also used the cases as enclosures for other projects - pretty handy if the form factor works! Here’s a passive EQ using an M68 case and some of the pots & transformers (designed to plug into a mic preamp for makeup gain).

Re: Shure M68 mixer
« Reply #5 on: July 13, 2019, 05:20:48 PM »
 ;D
Monkey and 20Trees  mucho gracias for the input ,

its curious isnt it , back in the day where silicon was costly ,much of the gain lifting was done in the transformer . ,now the transformer costs hundreds of times the sillycon  :(

These transformers are probably still good for lifting dynamic mics to tube grid ( the limitations of the capsule will eclipse the bandwidth of the transformer ), 10-20 euro-bucks might be worth spending just for the cores and mu metal screens for experimentation purposes
« Last Edit: July 13, 2019, 06:07:59 PM by Tubetec »

Whoops

Re: Shure M68 mixer
« Reply #6 on: July 14, 2019, 05:54:51 AM »
Those transformers are 1:10,
they were used in the Shure Level Loc also, theres threads on the Level loc diy project around here

They're small transformers, I don't know what the specs are but they shouldn't be great. Low frequency response is probably not good and with loads of distortion in the low end

cyrano

Re: Shure M68 mixer
« Reply #7 on: July 14, 2019, 09:57:24 AM »
They're small transformers, I don't know what the specs are but they shouldn't be great. Low frequency response is probably not good and with loads of distortion in the low end

I've experimented a bit with these and that's what I found. Don't put much signal in them and they're useable. Haven't found anything I can use the M68 I've got lying around yet. Not even for some "grit", as it is very noisy. Lots and lots of hiss...
Why is it people love to believe and hate to know?

Re: Shure M68 mixer
« Reply #8 on: July 14, 2019, 11:26:51 AM »
1:10 is probably more preferable than the higher ratio units for the modern age ,
I wonder what the potting material is , looks like silicone which would probably be easy to hack apart .
They should be good enough for dynamic mics as they are ,but it would be interesting to see how they handle a guitar cab mic'd with a 58 . The quality of the Mumetal sheilds looks excellent ,you just dont find that thickness of material in the modern day ,or if you did it would cost a lot.

Re: Shure M68 mixer
« Reply #9 on: July 14, 2019, 02:02:51 PM »
Those transformers are 1:10
how did you arrive at that turns ratio ?
were you in the loop with the 1:7  API 2622   ?


Whoops

Re: Shure M68 mixer
« Reply #10 on: July 14, 2019, 06:48:39 PM »
how did you arrive at that turns ratio ?
were you in the loop with the 1:7  API 2622   ?

Just different internet sources in the past, when I was doing my DIY Level loc,
I never measured those transformers myself

TwentyTrees

Re: Shure M68 mixer
« Reply #11 on: July 14, 2019, 09:25:08 PM »
They give about 20dB gain, which I understand to be 1:10.


 

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